I love the idea, and proposed 5 years ago.
FIrst of all, there's not going to be a system that won't be expandable. What you're asking a company to do is not make any more money and not try to make their development money back.
But as I proposed in another thread, a simple system that starts off with 3 or 4 classes and then is expanded on as advanced classes in the form of feats would great.
Classes are nothing more than what they are, classes. There's no level progression or anything of the sorts. There's no skills or preset save.
Instead, each level, players cna get a feat that expands the class. The feat might include a bonus to the ability modifier when used for a particular skill, a bonus to a particular save. The total bonus for anything is limited by level.
So in the core book, you get 3 or 4base classes (and thats it forever) and contains advanced class feats of about 7 to 11.
Doing like this, the book would be 100 pages tops and finally something i can hand someone and say, see its easy to learn. The one thing that got me into this was that the book wasn't daunting, it was cheap (20 bucks) and someone made my character for me, which alllowed me to learn hte rules.